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Other than the difficulties of traveling with a very pregnant woman so no attention for a bunch of rowdy kids along to the census accessor, I have no idea. The same quandry would apply for the story of Jesus at 12 getting left behind at the Temple ... no other children are mentioned in that story either.

LUKE 2:
41 Now his parents went to Jerusalem every year at the feast of the Passover.
42 And when he was twelve years old, they went up according to custom;

It seems that 12 was the "magic" age at which one participated in the Passover. Any younger children at home would not be attending the Passover.

Further---------

Luke 2:
Luke.2 [1] In those days a decree went out from Caesar Augustus that all the world should be enrolled.
[2] This was the first enrollment, when Quirin'i-us was governor of Syria.
[3] And all went to be enrolled, each to his own city.
[4] And Joseph also went up from Galilee, from the city of Nazareth, to Judea, to the city of David, which is called Bethlehem, because he was of the house and lineage of David,
[5] to be enrolled with Mary, his betrothed, who was with child.

There were either no other children of Joseph involved or they all had reached the age of majority and were on their own.

However, the same "brothers were traveling with His Mother while Jesus preached.

Luke 8:20 And he was told, "Your mother and your brothers are standing outside, desiring to see you."

Simple math would make these "older sons of Joseph" in their late 40's or considerably older during His Ministry.

Assuming Jesus was the "first born" of Mary and Joseph there would be no other children to be enrolled in the census.